A titanium oxide is used as a photocatalyst or a catalyst involved in an oxidation-reduction reaction. Especially, it is known that it can also be used as an electrode catalyst of a fuel cell by utilizing the oxygen reduction catalytic capacity of a titanium oxide catalyst.
In Patent Document 1, it has been reported that by heat-treating a metal carbonitride or a metal nitride in the presence of oxygen and hydrogen to create an oxygen defect in which oxygen is replaced with another element, an active site and electroconductivity can be secured, and therefore a titanium oxide catalyst having high oxygen reduction catalytic capacity can be produced.
In Patent Document 2, it has been reported that an oxide catalyst with high oxygen reduction catalytic capacity can be produced by sputtering a metal oxide such as TiO2 to prepare an oxygen reduction electrode having an oxygen defect for a direct fuel cell.
In Non Patent Document 1, it has been reported that a titanium oxide catalyst having high oxygen reduction catalytic capacity can be produced by treating a titanium oxycarbonitride in an atmosphere of a hydrogen, oxygen, and nitrogen, and thereafter treating it with an ammonia gas. Further, a powder has been prepared by heat-treating a titanium oxide having an anatase titanium dioxide structure in an ammonia gas atmosphere, and used as a reference in a comparison of oxygen reduction catalytic capacity.
The method of Patent Document 1 obtains an active site by replacing oxygen with another element, and is characterized that the crystal lattice is expanded when an oxygen defect is created. Therefore, the catalyst according to Patent Document 1 is unstable in the strongly acidic condition during a fuel cell operation, and is likely to be eluted, which is not preferable in terms of durability.
The method of Patent Document 2 prepares a catalyst in which oxygen atoms inside the metal oxide are decreased without replacement with another element, and does not prepare a catalyst with an oxygen defect generated by replacement with nitrogen. Meanwhile, since it is prepared as a thin film by sputtering, it is difficult to obtain a necessary amount for a catalyst having a large specific surface area such as a powder, which is not preferable.
Although by the preparation method of titanium oxycarbonitride according to Non Patent Document 1 an active site is obtained by replacing oxygen with another element, a strain tends to be generated in the crystal lattice, because carbon is contained in the catalyst in addition to titanium, oxygen, and nitrogen to increase the kinds of elements having different atomic radii. Therefore, the catalyst according to Non Patent Document 1 is unstable in the strongly acidic condition during a fuel cell operation, and is likely to be eluted, which is not preferable in terms of durability. Further, with respect to the ammonia-treated anatase titanium oxide for a reference, since a preparation method is general, the signal intensity ratio N—Ti—N/O—Ti—N in an X-ray photoelectron spectroscopic analysis exceeds 0.70, indicating a high titanium nitride content. As a result, the catalytic activity is lowered, and the spontaneous potential is also about 0.4V.